CD100, also known as semaphorin 4D (SEMA4D), is a transmembrane protein (e.g., SEQ ID NO: 1 (human); SEQ ID NO: 2 (murine)) that belongs to the semaphorin gene family. CD100 is expressed on the cell surface as a homodimer, but upon cell activation CD100 can be released from the cell surface via proteolytic cleavage to generate active sCD100, a soluble form of the protein. See Suzuki et al., Nature Rev. Immunol. 3:159-167 (2003); Kukutani et al., Nature Immunol. 9:17-23 (2008).
CD100 was first identified by generating two mouse monoclonal antibodies, BD16 and BB18, against activated human T cell clones (Herold et al., Int. Immunol. 7:1-8 (1994)). CD100 was the first example of a semaphorin expressed in the immune system. CD100 is expressed abundantly on the surface of resting T cells, and weakly on resting B cells, monocytes, and professional antigen-presenting cells, such as dendritic cells (DCs). Cellular activation can stimulate up-regulation of surface expression of CD100 on B cells and DCs, as well as the generation of sCD100. CD100 is thought to function as both a receptor, which signals through its cytoplasmic domain, and as a ligand (Hall et al., PNAS 93:11780-11785 (1996)). One of the receptors identified for CD100 is Plexin-B1. Plexin-B1 is expressed in non-lymphoid tissues and is a high affinity (1 nM) receptor for CD100 (Tamagnone et al., Cell 99:71-80 (1999)).
CD100 is an important mediator of T cell and B cell activation. CD100 knockout (CD100−/−) mice have reduced antibody responses to T-dependent antigens and impaired T cell priming. Both of these functions are restored upon the administration of sCD100 (Shi et al., Immunity 13:633-642 (2000)).
In addition to the demonstrated effects of CD100 on immune cells, CD100 also appears to play a direct role in the demyelination and axonal degeneration seen in neuroinflammatory diseases. The pathogenesis of inflammatory demyelinating diseases, such as MS, includes both an inflammatory phase involving immune cells as well as phases of selective demyelination and neurodegeneration. CD100 is expressed in central nervous system (CNS) oligodendrocytes and is an inhibitor of axonal regeneration. CD100 expression is up-regulated in oligodendrocytes at the periphery of spinal cord lesions (Moreau-Fauvarque et al., J. Neuroscience 23:9229-9239 (2003)). Culturing chronically activated T cells expressing sCD100 with human multipotent neural precursors or primary oligodendrocytes from rat brain induces apoptosis and process extension collapse (Giraudon et al., J. Immunol. 172:1246-1255 (2004); Giraudon et al., NeuroMolecular Med. 7:207-216 (2005)). CD100 induced apoptosis of neural precursors can be inhibited by the BD16 anti-CD100 antibody.
CD100 knockout mice are resistant to the development of experimental allergic encephalomyelitis (EAE), which is a mouse model for human multiple sclerosis (MS) (Kumanogoh et al., J. Immulol. 169:1175-1181 (2002)).
A number of other studies have demonstrated that CD100 induces growth cone collapse in neurons, and, in further support of the functional relevance of CD100 in neuroinflammation, it has been reported that there are highly elevated levels of sCD100 in cerebrospinal fluid (CSF) of HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients. Thus, there is a direct deleterious effect of sCD100 on oligodendrocyte and neural precursor integrity and CD100 may play a pathogenic role in demyelination. As an important mediator of both inflammatory responses and direct demyelination, there is a need in the art for CD100 neutralizing molecules, e.g., anti-CD100 antibodies, for treatment of inflammatory and demyelinating diseases.
CD100 is also a potent pro-angiogenic molecule. Activation of Plexin-B1 through CD100 binding transactivates c-Met and promotes the invasive ability of tumor cells and promotes angiogenesis both in vitro and in vivo. Immunohistochemical analysis of CD100 in a large tumor sample collection revealed that CD100 overexpression is a very frequent event in head and neck, prostate, colon, breast, and lung cancers.
CD100/Plexin B1 signaling has also been shown to induce migration of endothelial cells and to promote migration of tumor cells (Conrotto et al., Blood 105:4321-4329 (2005); Giordano et al., Nature Cell Biology 4:720-724 (2002)). CD100 induced endothelial cell migration is prevented by CD100-blocking antibodies and by CD100 knockdown. Knocking down CD100 expression in head and neck squamous cell carcioma (HNSCC) cells with CD100 short hairpin RNA (shRNA) before grafting into nude mice caused a dramatic reduction in tumor vascularity and tumor growth (Basile et al., PNAS 103:9017-9022 (2006)). Reports have recently pointed to a close correlation between inflammatory infiltration of the tumor stroma and a high vascular grade. CD100 is produced by inflammatory cells present in the tumor microenvironment. In an environment lacking CD100, the ability of mouse breast cancer cells to originate tumor masses and metastases was severely impaired, and the source of CD100 was tumor associated macrophages (Sierra et al., JEM 205:1673-1685 (2008)). Thus, there is a further need in the art for CD100 neutralizing molecules, e.g., anti-CD100 antibodies, for the treatment of CD100 cancer.